1. Field of the Invention
The present invention relates to a method and apparatus for the inspection of ceramic parts for structural and electrical defects therein, and, in particular for the inspection of posistors under electric loading.
2. Description of the Prior Art
Ceramic structural members, such as power posistors must possess a certain resistance to switching transients. Generally, a customer prescribes a specific switching arrangement and testing voltage and requires that the posistor survive a high number of switchings under these conditions without damage to the part.
Conventional methods of testing posistors apply a switching voltage to the posistor and the resistance of the posistor is measured before and after the application of voltage. If the resistance is increased after the application, it is assumed that mechanical defects were generated in the posistor during the electrical loading or that pre-existing defects were magnified thereby. This method has the disadvantage that minute defects which may lead to a gradual destruction of the part after a large number of switchings under testing voltage are not revealed. This is beause such minute defects will not measurably alter the electrical characteristics of the posistor, which will appear normal despite the presence of such minute defects. Only after a repeated number of switchings do such minute defects increase to a point such that an immediate destruction of the ceramic during switching occurs, or gradual destruction of the part occurs.
Some methods for inspecting for mechanical defects are known which utilize ultrasonics. Such processes still provide no definitive information as to the resistance of a particular part to switching transients because of the natural porosity of the ceramic and, again, because of the ofter minute extent of the defects. The same is true for currently employed electrical attenuation measurements, with which good results can be attained only in special cases.
All of the above methods also have the common disadvantage that detection of only mechanical defects is possible thereby. The greater number of switching failures is caused by resistance inhomogeneities in a mechanically fault-free ceramic part.